U.S. Pat. No. 4,540,359 discloses an injection molding machine which includes an injection mechanism driven by means of an electric motor rather than a hydraulic means. To control back pressure of the inspection plungers, this patent provides of an electrically-operated brake.
The use of an electrically-operated brake, makes it necessary to provide several components including a screw shaft for forwardly moving the injection plunger, a movable member threadedly engaged with the screw shaft to convert the rotational force into axial thrust of the injection plunger, and related components which are provided between the plunger and the back pressure control device. As a result of this structure backward force from the injection plunger received by the back pressure control device is indirect and the frictional force between the recited members degrades the accuracy with which the back pressure can be controlled.